The rapid development of the semiconductor industry mainly benefits from the improvement of the microfabrication technique of the microelectronic technology, while the lithography technique is one of the most critical techniques in the chip fabrication. Since the optical lithography technique continuously innovates, it breaks through the optical exposure limit desired by people, which makes it become a mainstream technology of current exposure.
The lithography system mainly includes four parts of illumination system (light source), mask, projection system and wafer. Diffraction occurs when light enters on mask, the diffracted lights after entering the projection system occur interference and image on the wafer, and after being subjected the development and etching processing, a pattern is transferred into the wafer.
In the actual production, it often confronts the case where it is required only a small part of pattern with mask sparse, and when there is only sparse process pattern on the mask plate, it may cause difference between an isolated imaging and a dense imaging during the lithography. Therefore, the Sub Resolution Assist Feature (SRAF) design is introduced at a process node of 90 nm in lithography. The sub resolution assist feature is added around a part of sparse pattern, however the sub resolution assist feature only acts as the functions of blocking and scattering lights in the exposure process to increase the focusing depth of the lithography pattern and enlarge the process window of the lithography, they can't form pattern on the wafer. Therefore, the existing technique adds sub resolution assist lines less than the minimum resolution of the lithography process on the mask plate.
These sub resolution assist lines include various serifs and scattering bars. The width of the serifs and scattering bars and the distance of them from the main feature pattern are more important, and need to be optimized according to the specific situations, to achieve the profile adjustment on the space image by influencing the phase spectrum with the scattering bars. These scattering bars or serifs, by improving the energy and phase distribution of various frequency components in the pattern spectrum, can effectively adjust the light intensity distribution of the space image without forming pattern on the etching resist, and can act as the functions of improving the linewidth deviation, strengthening the corner profile and increasing focusing depth of exposure.
The conventional design method of the SRAF is shown in FIG. 1, and specifically comprises the following steps of:
(1) depositing a metal layer on a surface of a mask plate;
(2) defining a sub resolution assist feature and a process pattern on the metal layer, and performing exposure;
(3) etching a portion other than the sub resolution assist feature and process pattern on the metal layer, to form the sub resolution assist feature and the process pattern which are located on the same horizontal plane.
In the procedure that the mask plate fabricated according to the above process is used as a mask, since the sub resolution assist feature and the process pattern are located on the same horizontal plane, the focusing planes of the sub resolution assist feature and the process pattern are also located on the same horizontal plane correspondingly, as shown in FIG. 2. This results in that the sub resolution assist feature may cause a ghost defect on the silicon wafer during the exposure, thereby influence the exposure quality of the silicon wafer.
In addition, the size of the sub resolution assist feature can't be too small either, and if the feature is too small, it can't effectively block the incident light, and may even deteriorate the process window of the lithography pattern. While in the procedure of the above process fabrication, if sub resolution assist feature is made become small, this will increase the cost of the mask lithography, thereby increase the fabrication cost of the mask plate. Therefore, in the procedure of the sub resolution assist feature design, the size of the sub resolution assist feature shall be appropriately controlled.